At my school 20-25 is typical per interview team, though some big firms have multiple teams interviewing in different rooms. One firm had 3 rooms (60 people), while another firm (IP) came on campus to interview 3 people... Unless you're at a top school, assume that you need to be in the interviewer's top 3-5 students to get a callback. The only time this might change is if your school is the main feeder for a firm (ie, Greenberg Traurig, Holland & Knight, Carlton Fields all recruit the majority of their summer classes at UF).
I often ask this question casually as I'm leaving an interview, generally phrased as "so where do we go from here?" or "what do you expect your hiring process to look like at this point?" and I've often gotten them to say how many slots the firm is planning to call back as well as the time frame to hear from them.